If Trainer difficulty is set to 100 for me and set to 50 for Joe and we are racing up a mountain and puttng out the same watts, who reaches the top quicker?
Logic is telling me that Joe would drop me and win the race to the top. Surely same watts on half the incline wins… I think haha
Many thanks
You both would reach the top at the same time. You will just use different gears.
Insert can of worms here, open and slowly back away. 
The trainer difficulty only affects the ‘gradient’ sent to the trainer. But in the game you would both still be climbing the same gradient at the same watts (and presumably the same weight), so you would both reach the top at the same time, in theory. Effectively, for a climb, reducing the trainer difficulty is the same as having larger sprockets on your cassette, making keeping a higher cadence possible without actually having to swap out your gearing.
You both finish at the same time (assuming your length and height are the same)
But for you it feels like alpe d’Huez (with grinding?)
And for Joe it feels like a long road on a slight incline. (With spinning to winning)
So to take this at a slightly different angle. If Gordy is set to 100 and 50 for Joe (as per the original example), and they output the same wattage then they reach the top at the same time. Ok.
But, who would find it more difficult to keep that same wattage and likely tire first and drop their wattage?
I assume the person (Joe) on 50 would be able to keep the same wattage for longer and therefore have a small advantage?
If two people are keeping the same wattage why would one tire before the other?
Load, basically, if two people of same height, weight, etc, are climbing a hill IRL at the same speed (and therefore putting out the same power), but one is pushing a 39x19 and the other is using a 53x17 it is likely that the person pushing the higher gear will tire faster due to putting a more severe load on their muscles. Power is a factor here, but not the only factor.
I’m trying to learn as I go.
If both people in your example were doing a flat TT in the same gears (Edit: as in your example) would the one pushing the higher gear tire faster?
It would also depend on what gears they have on their bikes. If the person riding at 100% TD had mountain bike gearing while the person at 50% had road racing gears on their bike, they might have a pretty similar experience in terms of how much they can spin depending on what their average wattage is.
If they both had the exact same gears, and one was grinding, and one was not, then yeah, the person grinding would probably burn their legs out first.
I think I’m trying to move the point away from trainer difficulty to how do you wish to spin your legs.
I struggle to pedal at more than around 75-85. So whether it is going along on the flat or riding up a hill personally I prefer (and think I go faster) to ride a heavier gear slower for more speed.
But the question posed is do I blow up sooner because I’m pushing that heavier gear?
I do not know what this means.
I think the quick answer is that individual physiologies are different and ‘ideal’ cadence varies. But in your case instead you had to push 65 while the other rider was able to push their normal ‘ideal’ (eg. say 85) then yes, you might blow up sooner. unless of course you’re more fit… 
There is no optimal cadence for all riders so you’ll have to figure it out for yourself. For me it’s definitely true that maximizing power on shorter efforts (not sprints) involves lower cadence. For longer rides I generally start faster and my cadence gradually drops after I get tired. My goal in the longer rides is to preserve my legs by recruiting my aerobic system more.
Hard to say. I think there is a balance. If you’re pushing too low a gear and spinning like a madman it is likely that you won’t overload your muscles, but you’ll still wear them out, especially if you’re not accustomed to high RPMs for an extended period of time. I think the key is to find the cadence that feels comfortable to you, and then change gears accordingly to stay in that range. You don’t want your ‘tachometer’ running in the red constantly, but you also don’t want it at the other end of the dial all the time.
As I said can of worms.
If they have good gears they can do the same power at the same power and the will be at the top feeling the same.
Nigel
Sorry I’m not being clear. You gave an example of using different gears to climb a hill and then used the words
“it is likely that the person pushing the higher gear will tire faster”
I was just trying to ask the question if they both used the same gears on a flat TT course who would tire first?
Impossible to answer. If you’re fishing for an answer that says that the racer who always uses the lowest gear and rides with the fastest cadence always wins, you won’t get that (hopefully).
No I’m not looking for that answer.
I think what I’m trying to say is that the best way to ride a hill or flat road is to ride it at the cadence and power that suits you best.
IF (and a big IF) Zwift have it right Trainer Difficulty should have no bearing on how ‘hard’ it is to ride up ADZ it should all be about HOW you wish to ride up AdZ. For me it is TD 50-75% 65-70rpm but if others want TD 0-25% big ring 80+rpm I don’t care but I just hope when we reach the top at the same time we have both exerted the same energy.
Ha! Sorry, I misinterpreted what you were saying. If by ‘used the same gears on a flat TT course’ you mean that they have the same available gearing on their bikes, then, yes, I think that the person pushing the higher gear (presuming it is difficult for them to push that gear) will tire more quickly.
Again, though, there are a lot of factors in play.